Existing methods for identifying chemicals (fluids) involve various kinds of spectroscopy, chromatography and standard chemical analyses. Such methods are not applicable when the chemicals are located inside of sealed containers and analysis must be performed noninvasively. An example would be identifying chemical warfare agents inside munitions or other toxic liquids in various containers. In such situations, it is imperative that the investigators not be exposed to chemical hazards and that the integrity of the container is not affected by the measurements. Other situations include monitoring of industrial chemicals flowing through pipes or very clean pharmaceutical products kept in reaction vessels. In such situations, it is often desirable that the pipe or vessel is not penetrated.
In U.S. Pat. No. 5,767,407 for “Noninvasive Identification Of Fluids By Swept-Frequency Acoustic Interferometry” which issued to Dipen N. Sinha on Jun. 16, 1998, a method for rapid, noninvasive identification and monitoring of chemicals in sealed containers or containers where direct access to the chemical is not possible is described. External transducers introduce standing-wave ultrasonic vibrational pattern into a fluid enclosed in a container over a range of frequencies. Multiple ultrasonic acoustic properties (up to four) of a fluid are simultaneously determined. For example, the speed of sound, sound attenuation, the liquid density, and the frequency dependence of sound absorption in the liquid, or a subset of these, may be determined from which chemical compounds can be identified. When the acoustic transducers cannot be placed directly in contact with the fluid, the transducers may be placed on the outside of the container wall.
The swept frequency acoustic interferometry (SFAI) technique for fluid characterization requires physical contact between the acoustic transducers and the container or pipe wall which limits the value of the technique in situations where direct physical contact is neither possible nor desirable. For example, in bottling plants where high speed measurements are required, placing coupling gels on each bottle is prohibitive. Further, in situations where the container surface is contaminated, hot or radioactive, direct contact is not indicated. It is possible, in principle, to make non-contact measurements using electromagnetic acoustic transducers, but these only operate in nonmagnetic metallic containers and must have sufficient wall thickness for the generation of the sound waves.
Additionally, whenever a transducer is attached to a measurement system, the transducer itself may affect the measurement since the presence of a solid transducer attached to the wall of a container essentially provides a leakage path for sound that otherwise would bounce back and forth within the container. As examples, this leakage depends on the contact pressure of the transducer and the coupling gel used, and is difficult to quantify. Subtraction of the effect of the transducers has been reported. Each time the sound inside the container reaches the inside wall of a container, it now finds an alternate path to travel through the wall to the solid transducer instead of being reflected and slowly decaying due to losses by absorption in the liquid. It is this sound absorption in the liquid that is of relevance to measure.
In U.S. Pat. No. 6,186,004 for “Apparatus And Method For Remote, Noninvasive Characterization Of Structures And Fluids Inside Containers” which issued to Gregory Kaduchack et al. on Feb. 13, 2001, an apparatus and method for remote, non-contact evaluation of structures and containers at large distances (on the order of several meters) in air is described. The invention utilizes an air-coupled, parametric acoustic array to excite resonance vibrations of elastic, fluid-filled vessels and structural members (at frequencies less than 40 kHz), where a nonlinear mixing process in the air medium transforms highly directional, narrow beamwidth higher acoustic frequencies into lower acoustic frequencies suitable for vibrational excitation of common structures. Vibrations were readily detected using a laser vibrometer in a fixed position relative to the acoustic array. Interior fluid characterization was achieved by analyzing the propagation of the generated guided waves (for example, the lowest-order generalized antisymmetric Lamb wave, a0) which is guided by the circumference of the container. The a0 Lamb wave is in a class of guided waves which exhibit strong flexural vibrations near the resonance frequency of the container.